Italy's Luna Rossa has beaten Britain's INEOS Team UK twice on a day of match-racing at breakneck speeds in the America's Cup challenger series final to take a 4-0 lead in the first-to-seven-wins series.

For the first time in the regatta, the boats engaged in traditional tacking duels, familiar to any competitive sailor, in which the leading and trailing boats threw in tack after tack as one protected its lead and the other sought to overturn it.

But these aren't normal boats. The AC75 foiling monohulls attained speeds of more than 46 knots (85km/h) in a solid sea breeze on Sunday and the crews, exchanging tacks, threw them around like dinghies.

The right-hand side of the course, where the wind passed over the bulk of Waiheke Island, was favoured and by crossing the start line first in both races, Luna Rossa was able to defend the right with classic match racing tactics.

That involved tacking with the British boat Britannia on each upwind leg, while Team UK sought to eat into the Italian team's lead or force a mistake.

"A tacking duel in these boats is something we haven't seen before," Italian co-helmsman Francesco Bruni said.

In the first race, Luna Rossa didn't lead by more than 18 seconds and held on to win by 13 seconds.

In the second race, an error by Team UK in the start box, when Britannia first reared up, then fell off its foils, conceded an advantage Luna Rossa retained throughout the race.

It went on to win by 41 seconds.

Britain's hopes of winning back the America's Cup for the first time in 170 years - since it lost the trophy to the United States in the first-ever race in 1851 - are now hanging by a thread.

Team UK has to win seven of the remaining nine races in the 13-race series to win through to the America's Cup match against holder Team New Zealand.

But they will have to wait because races scheduled for Wednesday have been postponed.

New Zealand Prime Minister Jacinda Ardern announced a three-day lockdown in Auckland.

The move comes after the country's biggest city reported three new local COVID-19 cases.